Operation of traction motors, for example, in a railroad locomotive, involves several layers of control for both propel (drive) operation and retard (brake) operation. For example, while an operator may adjust a throttle, an upper level controller may call for acceleration or deceleration, and a lower level controller may adjust torque based on the call for acceleration or deceleration. Even though the relationships are easily calculated in an ideal case, implementation of control at the given levels may require different data related to the drive system operating environment in order to carry out the necessary controls. For example, when managing torque in an AC traction motor, knowledge of motor flux is necessary for properly controlling inverters that set motor voltage and phase. In an ideal condition, DC link (supply) voltage can be used to estimate flux. However, many real world conditions, including wheel slip and resistive grid drop out, contribute to changes in link voltage that can lead to substantial errors in flux calculation.
A typical vehicle AC drive system may include several traction inverters, each driving one or more motors, all connected to a common DC link. When the motors are operated in retarding modes, power is fed from the motors into the DC link and the generated power is commonly dissipated in a resistive grid. The DC link voltage in this mode of operation is related to the total power produced by all inverters/motors as they feed that power into the resistive grid. In general, this means that a higher torque or power produced by one inverter/motor will increase the DC link voltage, however there is not always a one-to-one relationship of each inverter's contribution to the net DC link voltage due a lack of consistency among other inverters that are also connected to the DC link. In order to maintain desired efficiency and torque accuracy, there is a requirement to accurately set the traction motor flux reference in retarding modes of operation. This flux reference can then be used in typical vector-control methods to set current and voltage targets for motor control. Ideally, this flux reference will vary with DC link voltage (among other inputs) in order to maximize the flux and therefore the efficiency.
Typically the flux reference is set based on a measurement of the DC link voltage. Particularly during braking, the flux reference affects quadrature axis current Iq, which in turn affects torque. Torque affects power and power affects DC link voltage. This circular path can lead to oscillations and instability as the reference will necessarily lag behind the measurement, causing differences between the actual flux and the flux reference.
Another approach is to set the flux reference based on the inverter torque reference. In a single-inverter system this will work well, but in a system where multiple inverters can operate at varying power levels on a common DC link the lack of a one-to-one relationship between torque and voltage makes this approach problematic.
It is necessary to develop a flux reference more indirectly so that the lack of information and inherent instability in traditional measurements can be avoided while responding correctly to torque requirements.